You'll have to open any port which is to be used as an incoming port that a connection needs to be accepted on. If you need to accept connections on 25, you'll need to open it as well.

Keep in mind, if your account is rather new, DigitalOcean does place a block on 25, IIRC. So unless that block has been removed for your account, it won't matter if you open it on the Droplet, it'll be blocked at the network level so the connection will drop.